The majority of international transport of goods takes place by means of containers 1. The containers are standard-shaped transport units in which goods are packed for the duration of transport. Typically, containers come in three different sizes of 20 feet, 40 feet or 45 feet in length. A container is about 2.5 meters wide.
The containers are handled in a container terminal (either in ports or inland) by particular container cranes, which include rail mounted gantry cranes (RMG cranes) and rubber-tyred gantry cranes (RTG cranes). A particular type of rail mounted gantry crane is a ship-to-shore crane used for lifting containers to be unloaded from a ship onto a quay and, correspondingly, for loading containers brought to a quay on board a container ship.
For handling containers, a specific container grapple is typically mounted on a container crane for gripping and lifting a container. The length of a container grapple may be altered according to the length of the container being handled.
A current aim is to automate the container cranes so as to make the work of a container crane operator easier and quicker, or the operator may be completely eliminated from the container handling machine, in which case the container handling machine operates without an operator, unmanned.
If the aim is to facilitate the operator's work, typically, then, a work phase in the work cycle is carried out automatically, controlled by a computer. The aim is then to speed up that particular work phase, to increase precision, reliability or safety, or merely to facilitate the operator's work.
If the operator is completely eliminated from the control cabin of the container handling machine and the container handling machine operates unmanned, a significant portion of the work phases of the container handling machine is then carried out automatically, controlled by a computer.
When the aim is to automate the operations of the container crane, one functionality to be automated is the automatic stacking of containers on top of each other and/or precise positioning of containers at desired locations on the ground. In such a case, a typically 5-cm precision is aimed at for the location of the container corners. If the container grapple is suspended from the container crane by means of lifting ropes or some other oscillating suspension, it is not possible to assume, due to wind and the asymmetric load and stretch of the lifting ropes, among other things, that the container grapple hangs at sufficient precision perpendicularly below the crane in a correct position.
A skilled person will understand that to place an upper container on top of a lower container at a desired location in the control of a computer, for instance, it is necessary to be able to measure accurately the sideways location, longitudinal location of the container grapple and the skew of the container horizontally and, if necessary, to guide the container grapple to the correct location. As is previously known, said locations and skew can typically be controlled for instance by stay control ropes mounted on the container grapple. A skilled person will understand that by mounting for instance four stay ropes at the corners of the container grapple in accordance with the prior art, it is possible to both move and turn the container grapple horizontally by adjusting the relative lengths of the control ropes. When automating a crane, it is possible to control especially the control ropes by means of a computer.
So that the upper container can be smoothly lowered on top of a lower container, for instance by using a computer, the precise elevation and longitudinal trim of the container being lowered should preferably be known to be able to adjust the trim to correspond to the lower container or ground by means of the lifting drum of the lifting ropes, and to slow down the lowering movement optimally just before the container touches the lower container or ground. A skilled person will understand that the evaluation of the longitudinal trim of the container, in particular, by measuring the skew of the lifting drums is unreliable due to the stretching of the lifting ropes and a possible unbalanced load in the container.
A prior-art system that is capable of determining the location and position of the container grapple consists of cameras mounted on the crane and active, infrared light-emitting beacons mounted on the container grapple. However, a drawback of this system is the high number and complexity of the cameras (at least two) and the beacons (at least three) mounted on the container grappling element, and consequently the high price of the equipment. In addition, the measuring accuracy of the elevation measurement and longitudinal trim is not sufficient for the applications described above.